1. Field of the Invention
The present invention relates to piezoelectric resonators, manufacturing methods therefor, and electronic components using the piezoelectric resonators, and more particularly, to a novel piezoelectric resonator which maximizes the effective use of the mechanical resonance of a piezoelectric member, a manufacturing method therefor, and electronic components containing the novel piezoelectric resonator, such as an oscillator, a discriminator, and a filter.
2. Description of the Related Art
FIG. 28 is a perspective view of a conventional piezoelectric resonator. A piezoelectric resonator 1 includes a single piezoelectric substrate 2 having, for example, a rectangular plate shape as viewed from above. The piezoelectric substrate 2 is polarized in the thickness direction. On two opposite major surfaces of the piezoelectric substrate 2, electrodes 3 are provided. When a signal is input between the electrodes 3, an electrical field is applied to the single piezoelectric substrate 2 in the thickness direction and the single piezoelectric substrate 2 vibrates in the longitudinal direction.
In FIG. 29, there is shown a piezoelectric resonator 1 in which electrodes 3 are provided on two opposite major surfaces of a single piezoelectric substrate 2 having a square plate shape as viewed from above. The single piezoelectric substrate 2 of the piezoelectric resonator 1 is polarized in the thickness direction. When a signal is input between the electrodes 3 in the piezoelectric resonator 1, an electrical field is applied to the single piezoelectric substrate 2 in the thickness direction and the single piezoelectric substrate 2 vibrates in a square-type vibration mode (in the plane direction).
To produce an electronic component using the piezoelectric resonator 1, the piezoelectric resonator 1 is mounted on an insulating substrate 5 on which pattern electrodes 4 are provided, as shown in FIG. 30. The center of the piezoelectric resonator 1, which serves as a node, is supported by a support member 6 formed on a pattern electrode 4 so as to not interfere with the vibration of the piezoelectric resonator 1. The support member 6 is made from an electrically conductive material and electrically connects a pattern electrode 4 to one electrode 3 of the piezoelectric resonator 1. The other electrode 3 of the piezoelectric resonator 1 is connected to the other pattern electrode 4 via a lead wire 7. A metal cap 8 is placed on the insulating substrate 5. Since the support member 6 supports only the center of the piezoelectric resonator 1, which serves as a node, the vibration of the piezoelectric substrate 2 is not damped and the characteristics of the piezoelectric resonator 1 are prevented from deteriorating.
These piezoelectric resonators are of an unstiffened type, in which the vibration direction differs from the direction of polarization and the electrical field. The electromechanical coupling coefficient of such an unstiffened piezoelectric resonator is lower than that of a stiffened piezoelectric resonator, in which each of the vibration direction, the direction of polarization, and the direction in which an electrical field is applied are the same. An unstiffened piezoelectric resonator has a relatively small frequency difference .DELTA.F between the resonant frequency and the antiresonant frequency. This leads to a drawback in which a frequency-band width in use is narrow when an unstiffened frequency resonator is used as an oscillator or a filter. Therefore, the degree of freedom and flexibility in resonator characteristics design is low in such a piezoelectric resonator and electronic components using the same.
The piezoelectric resonator shown in FIG. 29 uses the first-order resonance in the longitudinal mode. Because of its structure, the piezoelectric resonator of FIG. 29 also generates large spurious resonances in odd-number-order harmonic modes, such as the third-order and fifth-order modes, and in a width mode. To suppress these spurious resonances, some solutions have been considered, such as polishing, increasing mass, and changing the shape of the electrode. These solutions increase manufacturing cost.
In addition, since the piezoelectric substrate has a rectangular plate shape, the substrate cannot be made thinner without sacrificing required strength. Therefore, the distance between the electrodes cannot be reduced and a capacitance between terminals cannot be increased. This makes it extremely difficult to achieve impedance matching with an external circuit. To form a ladder filter by alternately connecting a plurality of piezoelectric resonators in series and in parallel, the capacitance ratio of the series resonator to the parallel resonator needs to be made large in order to increase attenuation. Because a piezoelectric resonator has the shape and structural restrictions described above, however, large attenuation cannot be obtained.
In the piezoelectric resonator shown in FIG. 30, large spurious resonances such as those in the thickness mode and in the triple-wave mode in the plane direction are generated. Since the piezoelectric resonator needs a large size as compared with a piezoelectric resonator using the longitudinal vibration in order to obtain the same resonant frequency, it is difficult to reduce the size of the piezoelectric resonator. When a ladder filter is formed by a plurality of piezoelectric resonators, in order to increase the capacitance ratio between the series resonator and the parallel resonator, the resonators connected in series must have an increased thickness and electrodes are formed only on part of a piezoelectric substrate to make the capacitance small. In this case, since the electrodes are only partially formed, the difference .DELTA.F between the resonant frequency and the antiresonant frequency as well as the capacitance is reduced. The resonators connected in parallel are accordingly required to have small .DELTA.F. As a result, the piezoelectricity of the piezoelectric substrate is not effectively used, and the transmission band width of the filter cannot be increased.
When an electronic component is produced using the piezoelectric resonator, it is necessary to use a lead wire to connect the piezoelectric resonator to a pattern electrode on the insulating substrate. This increases manufacturing steps and production cost. The pattern electrode is formed on the support member and the piezoelectric resonator is secured to the support member. Strict precision is required in placing the center of the piezoelectric resonator on the support member. If the support position shifts, the vibration of the piezoelectric resonator leaks and superior resonator characteristics cannot be obtained.